1. Field of the Invention
The present invention relates generally to the packaging of electronic components. More particularly, the present invention relates to a method of forming an image sensor package.
2. Description of the Related Art
Image sensors are well known to those of skill in the art. An image sensor included an active area, which was responsive to electromagnetic radiation. To avoid obstructing or distorting the electromagnetic radiation which struck the active area of the image sensor, it was important to avoid contamination, e.g., from dust, of the active area.
Image sensors were fabricated from a silicon wafer. More particularly, a plurality of image sensors were formed in a single silicon wafer. The silicon wafer was singulated, sometimes called cut-up or diced, to separate the image sensors from one another. However, during this wafer singulation, silicon shards were generated. These silicon shards had a tendency to contaminate and scratch the active areas of the image sensors. As a result, image sensors were damaged or destroyed, which undesirably decreased the yield. However, to reduce cost, it is important to have a high yield.
The singulated image sensor was then used to fabricate an image sensor assembly. In this assembly, the image sensor was located within a housing, which supported a window. Radiation passed through the window and struck the active area of the image sensor, which responded to the radiation.
In one prior art assembly, an image sensor was mounted to a printed circuit mother board. After the image sensor was mounted, a housing was mounted around the image sensor and to the print circuit mother board. This housing provided a hermetic like seal around the image sensor, while at the same time, supported a window above the image sensor.
As the art moves to smaller and lighter weight electronic devices, it becomes increasingly important that the size of the image sensor assembly used within these electronic devices is small. Disadvantageously, the conventional image sensor assembly described above required a housing to support the window and to hermetically seal the image sensor. However, this housing was relatively bulky and extended upwards from the printed circuit mother board a significant distance, e.g., 0.100 inches (2.54 mm) to 0.120 inches (3.05 mm) or more. As a result, the image sensor assembly was relatively large.
In the event that moisture was trapped inside of the housing, defective operation or failure of the image sensor assembly was observed. More particularly, the moisture had a tendency to condense within the housing and on the interior surface of the window. Even if the housing later dried out, a stain was left on the window. In either event, electromagnetic radiation passing through the window was distorted or obstructed by either moisture condensation or stain, which resulted in defective operation or failure of the image sensor assembly.
For this reason, an important characteristic was the temperature at which condensation formed within the housing of the image sensor assembly, i.e., the dew point of the image sensor assembly. In particular, it was important to have a low dew point to insure satisfactory performance of the image sensor assembly over a broad range of temperatures.
In accordance with the present invention, an image sensor assembly, sometimes called an image sensor package, includes an image sensor having an upper surface. The image sensor further includes an active area and bond pads on the upper surface. The upper surface includes a noncritical region between the active area and the bond pads. A step up ring is mounted above the noncritical region. Electrically conductive traces on the step up ring are electrically connected to the bond pads by bond wires.
In one embodiment, a window is supported above the active area by a window support. The step up ring has a central aperture and is mounted around the window such that the window is located in or adjacent the central aperture. An inner package body, e.g., formed of an encapsulant, fills the central aperture and encloses sides of the window, thus mechanically locking the window in place.
The image sensor assembly further includes an outer package body, e.g., formed of an encapsulant. The outer package body encloses the bond pads and the bond wires. The outer package body has outer sides coplanar with sides of the image sensor. Advantageously, the image sensor assembly is the size of the image sensor, i.e., the image sensor assembly is chip size. Since the image sensor assembly is chip size, the image sensor assembly is extremely well suited for use with miniature lightweight electronic devices, which require small and lightweight image sensor assemblies.
In accordance with an alternative embodiment, an image sensor assembly includes an image sensor having an upper surface. The image sensor includes an active area and a bond pad on the upper surface of the image sensor. A step up ring includes an electrically conductive interior trace on a lower surface of the step up ring. The step up ring is mounted to the image sensor by an electrically conductive bump between the bond pad and the interior trace, i.e., the step up ring is flip chip mounted to the image sensor.
Also in accordance with present invention, a method includes mounting a window above an active area on an upper surface of an image sensor. A bond pad is on the upper surface of the image sensor and a noncritical region of the upper surface of the image sensor is between the bond pad and the active area. The method further includes mounting a step up ring above the noncritical region. A trace on the step up ring is electrically connected to the bond pad, for example, with a bond wire.
In one embodiment, the step up ring includes a central aperture and is mounted around the window such that the window is located in or adjacent to the central aperture. The central aperture is filled with an encapsulant to form an inner package body. An outer package body is formed to enclose the bond wire between the trace and the bond pad.
In another embodiment, a method includes mounting a window above an active area on an upper surface of an image sensor, the image sensor comprising a bond pad on the upper surface. An interior trace on a lower surface of a step up ring is aligned with the bond pad. A bump is formed between the interior trace and the bond pad to mount the step up ring to the image sensor.
In yet another alternative embodiment, windows are mounted above active areas on upper surfaces of image sensors integrally connected together as part of an image sensor substrate. A sheet includes step up rings integrally connected together. The sheet is aligned with an image sensor substrate. The sheet is mounted to the image sensor substrate by bumps, wherein the windows are located in or adjacent central apertures of the step up rings.
These and other features and advantages of the present invention will be more readily apparent from the detailed description set forth below taken in conjunction with the accompanying drawings.